We actually started out this venture as a part time thing that has blossomed into a full time thing. We usually have approximately 850-900 items listed in our store at any one time. I'm not sure I'd recommend this unless you don't mind mess in your home, or hopefully you have a lot of space. We have tubs, tables, and shelves of merchandise throughout our home. Our dining room has been converted into an office/shipping room.
We find our merchandise to sell at garage sales, resale shops, auctions, and outlet stores in our immediate area. I've even found one deal on ebay where the lady lived near me, had good merchandise very cheap, and I purchased from her and picked it up myself to save shipping, and resold those items. You have to be persistent in looking for items, going to all sales, scouring the newspapers for deals. Usually then about 3-4 times a year we have garage sales ourselves to get rid of the items we've gotten at auctions that weren't worth reselling, or items that have been listed a long time and haven't sold.
Make sure you have a camera that takes nice clear pictures, set yourself up a good spot for picture taking, and crop and make them as clear and clean as possible. That is the first thing people see of your item and it's important they can see it clearly and from every angle.
We try to ship most of our items Priority mail as you can get all of your boxes free from the USPS, but we also remain flexible if people want a cheaper shipping method. We also combine shipping on numerous items and place this in our ads to get people to buy several items from us enabling them to save themselves money on shipping, and making more sales for ourselves.
Hopefully this helps!